“Flu season started about a month earlier than we usually see and it’s consistently picked up over the last few weeks,” says Tom Skinner, spokesman for the CDC. “In fact, most states are experiencing moderate or severe levels of flu activity.”

The individual flu strains can determine how severe and widespread a particular flu season can be. The types of influenza — A, B and C — can manifest in many different viral strains.

The strains’ characteristics and resistance can change from year to year, so the World Health Organization, the CDC and other institutions try to predict which strains will be most prevalent during the next flu season. A vaccine is then developed to protect against them.

The Northern Hemisphere’s 2012-2013 seasonal influenza vaccine protects against three strains – A (H1N1), A (H3N2) and B. This season’s dominant strain, H3N2, can cause more serious illnesses than the others.

“We tend to see more severe flu when we have this particular strain circulating,” Skinner says. “In years past, when we’ve seen this strain circulate, we tend to see more illness in children and the elderly.”

The CDC says the flu vaccine is well-matched to the 2012-2013 flu strains. But like any medicine, it is not always 100 percent effective. Depending on the individual, Skinner says, the flu vaccine is 60 to 70 percent effective in healthy people but could be less effective in the elderly and in people with underlying medical conditions.

“But it’s the best medicine against the flu and we encourage everyone over the age of 6 months to get vaccinated every year,” Skinner says. “It’s the single most important thing you can do to protect yourself.”

If people get the flu shot and still contract the virus, the vaccine could actually make symptoms milder.

“They may not have as severe an illness as someone who wasn’t vaccinated and came down with the flu,” Skinner says.

The biggest misconception about the flu vaccine, Skinner says, is that the shot can give someone the virus.

“That’s simply not true,” he says. “The vaccine is made with a weakened or killed flu virus, so it’s impossible for you to actually develop the flu from getting the vaccine.”

Ryanne Mayersak, an Emergency Medicine Physician at George Washington University Hospital, agrees.

“People can get muscle aches after getting it or soreness at the injection site, but receiving the flu shot does not mean you will actually get the flu,” she says.

She says anyone is at risk for getting the virus and it’s not too late to get vaccinated.

“It will minimize the possibility that you will have symptoms for a longer period of time,” Mayersak says.

It takes about two weeks for the human body to build up the antibodies created as a response to the flu shot.

The people most at risk for the flu are children, the elderly and people with weakened immune systems, such as cancer patients, asthmatics, diabetics, organ transplant patients and people with underlying medical conditions.

“We know that flu can be very, very serious, if not deadly, in those individuals,” Skinner says.

Pharmaceutical manufacturer Sanofi Pasteur, Inc. developed an alternative flu vaccine with four times the amount of antigen, or active ingredients. It is designed specifically for senior citizens.

Clinical trials show that people who are 65 or older have a stronger immune response after receiving the Fluzone High-Dose vaccine. However, it is not clinically proven to be more effective in preventing the flu, as compared to the regular flu shot.